In operations relating to drilling boreholes with casing or running casing into a previously drilled borehole, a casing string may be used with a bottom hole assembly connected at its bottom end. During these operations it may be desired to retrieve the bottom hole assembly from the casing string for replacement, repair or final removal. The bottom hole assembly may be sized to be retrievable through the casing string inner diameter.
In a traditional retrieval procedure, a release tool is conveyed downhole to manipulate and unlock a lock assembly on the bottom hole assembly. The release tool and the bottom hole assembly are then tripped either independently or, preferably, together to surface. Generally, a wireline is used to engage and pull the lock and bottom hole assemblies to surface.
A provision must be made for the wireline to be run through the casing to retrieve the BHA. It is often advantageous to circulate drilling fluid down the ID of the casing while the wireline is being run and the BHA recovered to ensure that any influx of formation fluids is circulated out of the well in a controlled manner. It is also advantageous to reciprocate the casing while the BHA is being recovered so that the casing does not become stuck in the borehole. The top drive and casing drive system must be attached to the casing in order for circulation and reciprocation of the casing to be accomplished.
The drilling rig used to drill with casing may be a specially designed rig that facilitates the efficient operation of the wireline for running and retrieving the drilling BHA. The rig also must be equipped with a wireline unit that is capable of handling the drilling BHAs. For rigs designed for casing drilling, this wireline unit may be provided as an integral part of the rig.
Access for the wireline is provided through the top of the swivel, which may be incorporated as an integral part of the top drive. Utilizing a split crown block and split traveling block may facilitate the wireline access through the top of the swivel. Split blocks are ones where the sheaves used for carrying the drilling line are divided into two groups spaced laterally apart. The split crown arrangement allows a wireline sheave to be hung at the crown of the rig so the wireline can be aligned with the central axis of the drillstring. The split traveling block provides room for a wireline stripper assembly and wireline BOP to be attached to the top of the swivel to prevent the pressurized drilling fluid from escaping around the wireline as it is being run into and pulled from the casing. In some situations, it may be sufficient to provide only a split traveling block as the fleet angle from having the crown sheave offset slightly from the central axis of the drillstring.
The drilling BHA may be quite heavy and weigh as much as 30,000 pounds. A large braided cable, for example ¾″ in diameter, may be required to support this much weight and the sheaves used with such a cable are relatively large in diameter, for example 30″ in diameter. It is important that the sheaves and wireline pressure control equipment be positioned so that the wireline can enter the casing along its central axis. Otherwise, the cable will exert lateral forces on the casing or other equipment and will quickly cut into the casing inner wall.
As will be appreciated, wireline retrieval processes are costly, time consuming and complex.